Chung Y.-W. Practical guide to surface science and spectroscopy

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2.4 ENERGIES AND SHAPES OF AUGER PEAKS

Consider the case of a free atom with discrete energy levels. As dis-

cussed previously, the Auger electron energy E

WXY

due to an Auger

WXY transition is given by

WXY

⫽ E

⫺ E

⫺

␾

(2.1)

where ␾ is the work function and E

is the binding energy of the

W-level for the neutral atom. But E

and E

are not the X and Y shell

binding energies for the neutral atom. When the W-level electron is

removed, the resulting species is a positive ion, so that electrons in the

X and Y shells are moving in a potential of a ⫹e ion. Because of

screening effects, the actual charge experienced by these electrons may

be less than ⫹e. One may approximate E

and E

⫽

(Z) ⫹ E

(Z ⫹ 1)

(2.2)

where Z is the nuclear charge of the atom and i ⫽ XorY.

XAMPLE.

Calculate the kinetic energies of all KLL Auger peaks

from a clean polycrystalline aluminum surface, with information pro-

vided below:

Binding energy of 1s (K) electrons of Al ⫽ 1559.6 eV

Binding energy of 2s (L

) electrons of Al ⫽ 117.7 eV

Binding energy of 2p(L

) electrons of Al ⫽ 73.1 eV

Binding energy of 2s(L

) electrons of Si ⫽ 148.7 eV

Binding energy of 2p(L

) electrons of Si ⫽ 99.2 eV

Work function of polycrystalline Al ⫽ 4.2 eV

OLUTION.

There are three possible KLL transitions, viz., KL

, and KL

. Let us first calculate all the required energy

levels:

⫽ 1559.6 eV

⫽ 0.5(117.7 ⫹ 148.7 ⫽ 133.2 eV

L23

⫽ 0.5(73.1 ⫹ 99.2) ⫽ 86.15 eV.

CHAPTER2/AUGER ELECTRON SPECTROSCOPY

Therefore,

E(KL

) ⫽ 1559.6 ⫺ 2 ⫻ 133.2 ⫺ 4.2 ⫽ 1289 eV

E(KL

) ⫽ 1559.6 ⫺ 133.2 ⫺ 86.15 ⫺ 4.2 ⫽ 1336.05 eV

E(KL

) ⫽ 1559.6 ⫺ 2 ⫻ 86.15 ⫺ 4.2 ⫽ 1383.1 eV.

UESTION FOR

ISCUSSION.

Using a highly monoenergetic inci-

dent beam of electrons, the width of a certain neon KLL Auger transition

is determined to be 3 eV. If the incident beam energy is allowed to

have an energy spread of 10 eV, how will this affect the Ne Auger peak

width?

In some cases, electrons from the valence band are involved, either

as core–core–valence (CCV) or core–valence–valence (CVV) Auger

transitions. Most valence bands exhibit certain structures, that is, the

density of occupied electron states varies across the whole band. Most

valence bands have widths on the order of 5–15 eV. Therefore, in CCV

and CVV transitions, one would expect (1) the Auger peak width ⬇

the width of the valence band, and (2) the Auger peak shape is character-

istic of the valence band structure from which it is derived.

XAMPLE.

Show that the energy width of a CCV Auger peak is

equal to that of the valence band.

OLUTION.

The Auger kinetic energy for a CCV transition is given

CCV

⫽ E

⫺ E

⫺

␾

The energy width ⌬E

CCV

⫽兩⌬E

兩, since all core levels are assumed

to be sharp, and the work function

␾

is a constant. Therefore, the

energy width of a CCV Auger transition is equal to that of the valence

band.

2.5 CHEMICAL STATE EFFECTS

When an atom is placed under different chemical environments, two

possible changes may result in the Auger spectrum:

(a) Shift of the Auger peak position. This is caused by shifts of

electron energy levels due to electron transfer to or away from the

atom.

2.6 INTENSITY OF AUGER ELECTRON EMISSION

(b) Change of the Auger peak shape. This can be caused by the

change in the valence band structure. Also, as the Auger electron exits

from the solid, it may undergo energy loss (or gain) collisions resulting

in satellite features. Change of chemical environments can affect energy

loss (or gain) mechanisms and thus the overall shape of the Auger

peak. These changes can be useful in studying chemical state of atoms

on surfaces.

2.6 INTENSITY OF AUGER ELECTRON EMISSION

Three intrinsic factors affect Auger electron intensity:

(a) Ionization cross-section. Auger transitions of a given type can

be initiated only after a given core electron is ionized (K-shell electron

in the case of a KLL Auger transition). This requires the incident

electrons to have a minimum (threshold) energy E

equal to or greater

than the binding energy of this core electron. Above E

, the incident

electrons can produce ionizations at a rate directly proportional to the

incident electron flux ⌽ and the number of atoms per unit volume n,

that is,

⫽ n

⌽␴

, (2.3)

where dn/dt is the rate of ionization per unit volume, and ␴ is known

as the ionization cross-section. A schematic plot of ␴ versus incident

electron energy is shown in Fig. 2.3. Typically, maximum ␴ is reached

FIGURE 2.3 Variation of ionization cross-section with incident electron energy.

CHAPTER2/AUGER ELECTRON SPECTROSCOPY

when the incident electron energy is 3 to 5 times the threshold ionization

energy.

The shape of this curve can be explained as follows. Clearly,

ionization will only occur when E ⬎ E

. When E increases, ␴ increases.

But when E ⬎⬎ E

, electrons are moving very fast and spend a

proportionally shorter time near individual atoms, giving rise to smaller

number of ionizations per unit volume and hence smaller ␴. Since

Auger transitions can only be initiated by first ionizing the atom,

one would expect the intensity of any given Auger peak to follow a

dependence similar to that shown in Fig. 2.3. Therefore, in typical

Auger studies where we are interested in Auger transitions due to inner

shells of ionization energies less than 1500 eV, the primary electron

energy is usually restricted to 5–10 keV. On occasions where high

spatial resolution is required, incident electron energies up to 30 keV

are used. In general, one would want to use the lowest possible electron

beam energy consistent with one’s requirement for optimum signal and

spatial resolution. For a given beam current, the amount of energy

deposited on the surface per unit time is proportional to the beam

voltage. Too much power may cause surface damage or accelerate

surface contamination.

(b) Auger yield. After an inner shell electron is knocked off, a

higher shell electron falls down to fill the vacancy. Following this,

there are two possibilities: Auger electron or X-ray emission. Defining

as the probability of Auger electron emission and P

as that of

X-ray emission, we have

⫹ P

⫽ 1. (2.4)

For instance, if one measures P

and P

due to K-shell vacancies, one

obtains results shown in Fig. 2.4. Note that P

starts to overtake P

for elements with atomic number greater than 32. This does not mean

that Auger electron spectroscopy is only useful for low atomic number

elements. For the purpose of surface analysis, we are interested in

Auger electron energies less than 1500 eV. For example, for elements

with atomic number from 19 (K) to 70 (Yb), we are interested in Auger

transitions derived from the filling of M-shell vacancies. In this energy

range, the Auger yield always dominates over the corresponding

X-ray yield. As a result, the relative Auger yields do not vary by more

than one order of magnitude for all elements in the periodic table that

give rise to Auger transitions of energies from 50 to 1500 eV. A plot

2.6 INTENSITY OF AUGER ELECTRON EMISSION

FIGURE 2.4 Auger and X-ray yields due to K-shell vacancies versus atomic

number.

of the relative Auger sensitivity versus atomic number at an incident

electron energy of 3 keV is shown in Fig. 2.5.

them undergo collisions in the surface region resulting in Auger electron

emission. The majority will penetrate through the surface without gener-

ating any Auger electrons. However, some of these electrons may

undergo single or multiple large-angle scattering bringing them back

to the surface region (backscattering). These electrons can initiate Auger

FIGURE 2.5 Relative KLL Auger sensitivities for elements at 3 keV primary

energy obtained from a cylindrical mirror analyzer.

CHAPTER2/AUGER ELECTRON SPECTROSCOPY

transitions if they have sufficient energy. This contribution to the Auger

signal depends on the energy of the incident electrons and the backscat-

tering probability, which is a function of the atomic number of the

specimen. The backscattering contribution is larger for heavier ele-

ments. For incident electrons of energy ⬇ 10 keV, electron backscatter-

ing can occur from a depth ⬇ 0.1 to 1 ␮m (Fig. 2.6), depending on

the atomic number.

Backscattering can lead to some interesting phenomena. Since back-

scattering is a function of atomic number, a light element in a heavy

element matrix will give a larger Auger signal than a light element in

a light matrix of the same concentration. For example, when one depos-

its a thin film of molybdenum onto a tungsten substrate and monitors

the molybdenum Auger peak at 120 eV, one obtains results shown in

Fig. 2.7. At small molybdenum thickness, the Auger signal increases

linearly with thickness, as expected. However, with further increase in

film thickness, the Auger signal reaches a maximum and then decreases

slowly to the steady state value of the bulk. The Auger signal at the

maximum is about 20% greater than that obtained from bulk molybde-

num. This is due to tungsten (Z ⫽ 74) having a larger backscattering

power than molybdenum (Z ⫽ 42). The backscattering contribution

from tungsten gives rise to the extra molybdenum Auger intensity at

120 eV.

FIGURE 2.6 Contribution to Auger electron emission from primary and backscat-

tered electrons.

2.8 SCANNING AUGER MICROPROBE

FIGURE 2.7 Auguer intensity due to molybdenum as a function of Mo thickness

on tungsten.

2.7 PROFILE ANALYSIS

Sometimes, one is interested in the depth distribution of elements. This

can be accomplished by removal of surface atoms via inert gas (usually

argon) ion sputtering and simultaneous analysis of the surface composi-

tion using Auger electron spectroscopy. However, the sputtering rates

of different components of the solid can be different. Surface composi-

tion can be affected by sputtering so that the concentration profile

obtained by sequential sputtering and Auger analysis has to be interpre-

ted carefully.

UESTION FOR

ISCUSSION.

Even when preferential sputtering is

absent (i.e., all elements are sputtered at the same rate), the sputter

Auger profile does not reveal the true composition profile. Why?

UESTION FOR

ISCUSSION.

Argon is the preferred inert gas for

sputtering. Why?

2.8 SCANNING AUGER MICROPROBE

High spatial resolution Auger analysis can be done by using a small

probing electron beam for exciting Auger transitions. The analysis can

CHAPTER2/AUGER ELECTRON SPECTROSCOPY

be done in a spot mode. Alternatively, one can scan the electron beam

across the sample surface and tune the electron analyzer to detect Auger

electrons from any selected element. This generates a picture of the

spatial distribution of any element on the surface (Auger element map-

ping). When the same electronics system is equipped with a secondary

electron detector or absorbed current amplifier, the scanning Auger

microprobe (SAM) functions as a scanning electron microscope. In

this way, one can correlate information obtained from Auger electron

spectroscopy and surface topography. Today’s state-of-art SAMs have

electron beam sizes ⬇ 50 nm and are very useful in solving a wide

range of materials problems requiring elemental analysis at the submi-

cron scale.

There are three major considerations in the use of SAM:

(a) Electron beam damage. When one focuses an electron beam

into a small spot, the energy flux can be very high and can result in

local heating. The consequence can range from no effect to accelerated

specimen contamination or destruction of the surface.

(b) Backscattering artifacts. Because of the diffusion of the pri-

mary electron beam in solids, the Auger signal can sometimes arise

from a position different from the original impact point of the primary

electron beam. This can limit spatial resolution.

collect electrons emitting in all directions. Therefore, shadowing can

result in some cases. Also, near edges (e.g., corners of grain boundary

faces), enhanced Auger signal will result even with a homogeneous

solid. One should be careful not to interpret this as due to higher

concentration of the corresponding element.

Both backscattering and topography artifacts can be partially cor-

rected by normalizing the Auger peak intensity to that of the secondary

electron background at the same energy. The assumption is that back-

scattering and the topography affect the intensity of Auger electrons

and secondary electrons of the same kinetic energy to the same extent.

XAMPLE.

In performing Auger point analysis, the following op-

erating conditions are used:

Beam area ⫽ 1 ␮m ⫻ 1 ␮m

Beam voltage ⫽ 10 kV

Beam current ⫽ 1 nA.

2.9 QUANTITATIVE ANALYSIS

Calculate the energy flux.

OLUTION.

Energy flux ⫽ energy/(area ⫻ time)

⫽ 10

⫻ 10

⫺9

/(10

⫺12

)

⫽ 10

W/m

This is about 10

times the solar energy flux!

2.9 QUANTITATIVE ANALYSIS

Auger electron spectroscopy is a popular analytical tool because it is

relatively easy to use. Interpretation is usually straightforward. Auger

derivative spectra are compiled for all elements in the periodic table.

Element identification is just a matter of matching peaks.

Quantitative analysis is more difficult. The fundamental goal of

quantitative analysis is to determine the functional relationship between

the measured Auger signal in the N(E)ordN/dE mode and the surface

composition of a given specimen. To explore this, we use a schematic

setup shown in Fig. 2.8 in which the primary electron beam E

impinges

on the specimen surface at normal incidence, and the emitted Auger

electrons are collected at an angle ␪ from the surface normal. Let us

break the whole Auger process into three discrete steps: ionization,

emission, and collection.

FIGURE 2.8 Auger electron generation from different atomic layers.

CHAPTER2/AUGER ELECTRON SPECTROSCOPY

(a) Ionization. At the ith atomic layer from the surface of the solid,

the number of ionization events per unit time dN

/dt is given by

⫽

␳

␴

r (2.5)

where I

is the incident electron current, ␳

the number of atoms per

unit area in the ith layer, x

the atom fraction of the target element in

the ith layer, ␴ the ionization cross-section, and r the backscattering

enhancement factor. We define r as the total number of ionizations in

the ith layer divided by the number of ionizations due to the direct

beam alone. We assume no significant attenuation of incident electrons

so that the beam current at the ith layer is still I

(b) Emission. The probability of emission is equal to P

(E,␪).

is the probability of Auger emission after ionization, and q

is the

probability that the Auger electron emitted at an angle ␪ from the normal

from the ith layer will escape without energy loss and is given by

(E,

␪

) ⫽ exp

冋

⫺

(i ⫺ 1)d

␭

cos␪

册

, (2.6)

where d is the spacing between layers and ␭ the mean free path of

electrons with energy E. Here, we assume that (i) each layer fully

covers the layer below it, (ii) surface roughness is negligible, and (iii)

is independent of azimuth angle. For single crystals, we know that

assumption (iii) is not valid. Auger electrons may undergo diffraction,

channel, or attenuate anisotropically in a single crystal. Note also that

the Auger electron may be refracted when exiting the surface because

of the potential difference between the solid and vacuum.

collects a maximum fraction ⍀/4␲ where ⍀ is the acceptance solid

angle of the analyzer. Because of scattering from grids or walls of the

analyzer, only a fraction T of the electrons entering the analyzer with

the appropriate pass energy will be transmitted through the analyzer

and be detected. T is a function of energy E. The total Auger current

at energy E is then given by

⫽ I

␴

⍀

␲

再

兺

␳

exp

冋

⫺

(i ⫺ 1)d

␭

cos

␪

册冎

. (2.7)

The summation is from i ⫽ 1 (topmost layer) to i ⫽ ∞. Note that ␳

r, d, and ␭ all depend on composition. The variation of these quantities